Biofuel production processes of preparing bioethanol from cellulose are grossly divided into feedstock acquisition, pretreatment, saccharification, fermentation and purification processes. Particularly, the pretreatment process is indispensable when non-degradable woody biomass composed of cellulose, hemicellulose and lignin, linked in a complex and hard form, is used. The pretreatment process aims to effectively separate cellulose and hemicellulose from lignin having a complex and hard structure, and the separated cellulose is hydrolyzed into glucose, a representative monosaccharide, by three enzymes, i.e., endo-β-1,4-glucanase) [EC 3.2.1.4], exo-β-1,4-glucanase [EC 3.2.1.91], and β-glucosidase (EC 3.2.1.21) in the saccharification process. When glucose is fermented, ethanol is produced.
A technology for enzymatic hydrolysis of cellulose and hemicellulose has been known long ago, but has disadvantages in that high catalyst costs are required, the reaction proceeds slowly, and high equipment investment is required due to the need for a large-sized reactor. In addition, when a large amount of an enzyme is used, the rate of saccharification can increase, but cost problems arise, and there is a limit to increasing the efficiency of saccharification, due to the inhibitory action of a reaction substrate and a reaction product. However, in this case, the extreme reaction conditions of high temperatures and high pressures are not required, no toxic byproducts are produced, and the consumption of energy is reduced. Thus, there is a high need for the development of a technology for saccharifying biomass using an enzyme.
In order to achieve the commercialization of cellulosic ethanol, cellulosic ethanol should be produced at a concentration of 5% (w/w) so that ethanol can be separated by distillation using a small amount of energy. For this purpose, a saccharification liquid having a sugar content of 11% (w/w) or more based on ethanol (theoretical yield: 0.51, and actual fermentation efficiency: 90%) should be produced. After pretreatment, the enzymatic saccharification of biomass having a solid content of about 15-20% (sugar content: 50-70%) should be started.
However, biomass having a solid content of 15-20% is in a state similar to kneaded clay or soybean paste, in which the movement of a fluid in the biomass substrate hardly occurs. Thus, it is not easy to distribute a saccharification enzyme uniformly in the biomass substrate having this solid content or to maintain a reaction temperature suitable for saccharification. In addition, because the pH of the saccharification liquid is reduced due to the dissociation of acetyl groups present in the biomass during the saccharification process, a basic substance (e.g., NaOH) should be continuously fed during the reaction, but is difficult to mix, and thus makes it difficult to maintain an effective saccharification reaction.
For the efficient mixing of biomass, a variety of stirred tank reactors have been developed, which perform fine crushing by perforated plates and mixing by stirring means.
EP 0011870A1 discloses a stirred tank reactor for mixing a solid material and a liquid material, which comprises fixed perforated plates and stirring means. U.S. Pat. No. 8,617,480B2 discloses a stirred tank reactor, which comprises fixed perforated plates and stirred means together with an injection nozzle disposed in a space defined by the perforated plates. U.S. Pat. No. 4,409,329A discloses mixing biomass using perforated plates, and is characterized in that the distance between the perforated plates for mixing of biomass having a high solid content is as extremely narrow as 5 cm or less and in that an additive is introduced through an inlet hole provided in the upper portion of the stirred tank reactor.
The discussion in the foregoing background section is to provide general background information, and does not constitute an admission of prior art.